This invention relates to curing agents for fluoropolymers and to curable fluoropolymer compositions. In another aspect, the present invention relates to delayed curing fluoropolymer compositions.
Fluorocarbon elastomers are synthetic elastomeric polymers with a high fluorine contentxe2x80x94see, for example, W. M. Grootaert et al., Fluorinated Elastomers, 8 KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY 990-1005 (4th ed. 1993). Fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers such as hexafluoropropene (C3F6), have become the polymers of choice for high temperature applications, such as seals, gaskets, and linings. These fluoropolymers exhibit favorable properties against the exposure to aggressive environments such as solvents, lubricants, and oxidizing or reducing agents. Additionally, these polymers can be compounded and cured to have high tensile strength, good tear resistance, and low compression set.
Presently used curing agents for fluoropolymers include aromatic polyhydroxy compounds, such as polyphenols, used in combination with certain vulcanization accelerators such as ammonium, phosphonium, or sulfonium compounds. U.S. Pat. Nos. 4,882,390 (Grootaert et al.); 4,912,171 (Grootaert et al.); and 5,086,123 (Guenthner et al.), for example, describe these compounds.
In accordance with conventional curing processes, desired amounts of compounding ingredients and other conventional adjuvants or ingredients are added to unvulcanized fluorocarbon elastomer stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as Banbury mixers, roll mills, or other convenient mixing device. The components and adjuvants are distributed throughout the fluorocarbon gum during milling, during which period the temperature of the mixture typically will not rise above about 120xc2x0 C. The curing process typically comprises either injecting (injection molding) the compounded mixture into a hot mold or pressing (compression molding) the compounded mixture in a mold, for example, a cavity or a transfer mold, followed subsequently by an oven-cure (post cure).
Many conventional fluoropolymer compositions tend toward xe2x80x9cscorchingxe2x80x9d behavior, that is, the premature crosslinking or partial cure of the composition when exposed to elevated temperatures or conditions of high shear. This scorching behavior particularly is pronounced when the fluoropolymer is injection molded, wherein scorching is characterized by a premature cure initiation occurring prior to and during injection of the compounded composition into a mold. This can cause non-uniform curing of the fluoropolymer and results in poor physical properties.
The point of cure initiation for injection-molded fluoropolymers may be defined as the time after which the compounded fluoropolymer is subjected to injection-molding conditions (that is, upon introduction into an injection barrel at a temperature of approximately 70-90xc2x0 C. and/or while injecting the compound into the mold under high shear at temperatures between about 180xc2x0 C. and 200xc2x0 C.) when the curing compound begins to gel or harden. Such a change in physical properties, particularly the corresponding viscosity increase, can greatly reduce processing efficiency by hindering the ability to inject the compounded mixture into a mold. Scorching phenomena also produce high levels of waste product; because a cured fluoropolymer is very difficult to reprocess, any fluoropolymer that cures outside the mold cavity must usually be discarded.
Thus, there exists a need for fluoropolymer curing agents that provide a composition having improved scorch safety and end-use products having improved physical properties.
In one aspect, the present invention provides curable fluoropolymer compositions comprising the reaction product of: (a) fluorine-containing polymer or blend of fluorine-containing polymers each comprising interpolymerized units derived from one or more fluorine-containing ethylenically unsaturated monomers; (b) organo-onium compound; and (c) oxalate-blocked crosslinking agent.
In another aspect, the invention provides a method of curing a fluoropolymer comprising the steps of: (a) mixing organo-onium compound and oxalate-blocked compound into said fluoropolymer to form a curable fluoropolymer composition, said onium and oxalate-blocked compounds present in a sufficient amount to crosslink said fluoroelatomer to the desired degree; and (b) heating the curable fluoropolymer composition at a temperature of from about 180 to 210xc2x0 C. for a sufficient time to crosslink said fluoropolymer.
In another embodiment, the invention provides a composition of matter comprising an oxalate-blocked compound having the formula: 
wherein
Z is an aryl or polyaryl group;
R is an aryl group or an alkyl group; and
n and nxe2x80x2 each is independently selected as 0 or 1 with the proviso that when either n or nxe2x80x2 is 0, its corresponding portion of the Z moiety is terminated by hydrogen (that is, its corresponding terminal portion is xe2x80x94Zxe2x80x94OH) or is terminated by a metal or nonmetal cation.
The combinations of an organo-onium compound and the oxalate curing agents of the present invention provide increased processing control in the curing of fluoropolymer compositions, and in the formation of articles derived therefrom, without adversely affecting the physical properties of those cured compositions and articles.
The use of oxalate-blocked crosslinking agent in accordance with the teachings of the invention, either alone or in combination with one or more other crosslinking agents, yields improved scorch safety of curable fluoropolymers by providing a retarded cure at pre-molding temperatures below about 150xc2x0 C. and a rapid cure at molding temperatures of about 180 to about 210xc2x0 C. The ability significantly to retard this curing mechanism outside of the mold (where the temperature of the admixture typically do not exceed 150xc2x0 C.) drastically reduces the probability of severe scorching behavior and consequently reduces attendant processing difficulties. For example, such ability allows for heating of the compound in a cold runner injection-molding process without scorching, thereby reducing the amount of waste generated while also reducing cycle times.
Among the polymers that may be compounded in accordance with this invention are generally the fluoropolymers whose interpolymerized units are derived from one or more of the following fluoromonomers: vinylidene fluoride, vinyl fluoride, hexafluoropropene, chlorotrifluoroethylene, 2-chloropentafluoropropene, fluorinated vinyl ethers, fluorinated allyl ethers, tetrafluoroethylene, 1-hydropentafluoropropene, dichlorodifluoroethylene, trifluoroethylene, and mixtures thereof. Said fluoromonomers may also be copolymerized with other compounds such as with other cure-site monomers (for example, bromine-containing monomers or perfluorinated monomers such as perfluorobenzyl vinyl ether) or with non-fluorinated alpha-olefin co-monomers (for example, ethylene or propylene). Preferred fluoropolymers are copolymers of vinylidene fluoride and at least one terminally ethylenically-unsaturated fluoromonomer containing at least one fluorine atom substituent on each double-bonded carbon atom, each carbon atom of said fluoromonomer being substituted only with fluorine and optionally with chlorine, hydrogen, a lower fluoroalkyl radical, or a lower fluoroalkoxy radical.
Fluoropolymer copolymers according to the type described above are available commercially as copolymer gumstock under, for example, the xe2x80x9cFluorelxe2x80x9d trademark by Dyneon LLC, Saint Paul, Minn. Suitable products of these lines include THV(trademark) 200 and Fluorel(trademark) FC-2230, FC-2145, FC-2178, and FC-2211. Other commercially available products include fluoropolymers sold under the xe2x80x9cVitonxe2x80x9d trademark.
The organo-onium compound which is admixed with the fluorine-containing polymer is capable of functioning as a vulcanization accelerator. As is known in the art, an organo-onium is the conjugate acid of a Lewis base (for example, phosphine, amine, ether, and sulfide) and can be formed by reacting said Lewis base with a suitable alkylating agent (for example, an alkyl halide or acyl halide) resulting in an expansion of the valence of the electron donating atom of the Lewis base and a positive charge on the organo-onium compound. Many of the organo-onium compounds useful in the present invention contain at least one heteroatom, that is, a non-carbon atom such as N, P, S, O, bonded to organic or inorganic moieties. One class of quaternary organo-onium compounds particularly useful in the present invention broadly comprises relatively positive and relatively negative ions wherein a phosphorus, arsenic, antimony or nitrogen generally comprises the central atom of the positive ion, and the negative ion may be an organic or inorganic anion (for example, halide, sulfate, acetate, phosphate, phosphonate, hydroxide, alkoxide, phenoxide, bisphenoxide, etc.).
Many of the organo-onium compounds useful in this invention are described and known in the art. See, for example, U.S. Pat. Nos. 4,233,421 (Worm); 4,912,171 (Grootaert et al.); 5,086,123 (Guenthner et al.); and 5,262,490 (Kolb et al.), all of whose descriptions are herein incorporated by reference. Representative examples include the following individually listed compounds and mixtures thereof:
triphenylbenzyl phosphonium chloride
tributylallyl phosphonium chloride
tributylbenzyl ammonium chloride
tetrabutyl ammonium bromide
triaryl sulfonium chloride
8-benzyl-1,8-diazabicyclo [5,4,0]-7-undecenium chloride
benzyl tris(dimethylamino) phosphonium chloride
benzyl(diethylamino)diphenylphosphonium chloride
Another class of organo-oniums finding utility in the practice of this invention include acid-functional oniums that can represented by Formula I below. 
wherein:
Q is a nitrogen, phosphorus, arsenic, or antimony;
Z may be a substituted or unsubstituted, cyclic or acyclic alkyl group having from 4 to about 20 carbon atoms that is terminated with a group of the formula xe2x80x94COOA where A is a hydrogen atom or is a metal cation or Z is a group of the formula CY2xe2x80x94COORxe2x80x2 where Y is a hydrogen or halogen atom, or is a substituted or unsubstituted alkyl or aryl group having from 1 to about 6 carbon atoms that may optionally contain one or more catenary heteroatoms and where Rxe2x80x2 is a hydrogen atom, a metal cation, an alkyl group, or is an acyclic anhydride, for example, a group of the formula xe2x80x94COR where R is an alkyl group or is a group that itself contains organo-onium (that is, giving a bis organo-onium); preferably, Rxe2x80x2 is hydrogen; Z may also be a substituted or unsubstituted, cyclic or acyclic alkyl group having from 4 to about 20 carbon atoms that is terminated with a group of the formula xe2x80x94COOA where A is a hydrogen atom or is a metal cation;
R1, R2, and R3 are each independently an alkyl, aryl, alkenyl, or any combination thereof; each R1, R2, and R3 can be substituted with chlorine, fluorine, bromine, cyano, xe2x80x94ORxe2x80x3 or xe2x80x94COORxe2x80x3 where Rxe2x80x3 is a C1 to C20 alkyl, aryl, aralkyl, or alkenyl, and any pair of the R1, R2, and R3 groups can be connected with each other and with Q to form a heterocyclic ring; one or more of the R1, R2, and R3 groups may also be group of the formula Z where Z is as defined above;
X is an organic or inorganic anion (for example, halide, sulfate, acetate, phosphate, phosphonate, hydroxide, alkoxide, phenoxide, or bisphenoxide); and
n is a number equal to the valence of the anion X.
Another class of useful organo-onium compounds include those having one or more pendent fluorinated alkyl groups. Generally, the most useful such fluorinated onium compounds are disclosed in U.S. Pat. No. 5,591,804 (Coggio et al.). Representative of this useful class of onium compounds are the following: 
Useful oxalate blocked compounds used as crosslinking agents in accordance with the present invention have the formula: 
wherein Z is an arylene or polyarylene group, and is preferably a polyphenylene group of the formula: 
wherein
A is a difunctional aliphatic, cycloaliphatic, or aromatic radical of 1 to 13 carbon atoms, or a thio, oxy, carbonyl, sulfonyl, or sulfonyl radical, A is optionally substituted with at least one chlorine or fluorine atom, x is 0 or 1;
R is an aryl group or an alkyl group; and
n and nxe2x80x2 each is independently selected as 0 or 1 with the proviso that when either n or nxe2x80x2 is 0, its corresponding portion of the Z moiety is terminated by hydrogen (that is, its corresponding terminal portion is xe2x80x94Zxe2x80x94OH) or is terminated by a metal or nonmetal cation.
Preferably, A is a difunctional aliphatic radical or a difunctional perfluoroaliphatic radical.
Oxalate-blocked compounds useful in the formulations described above wherein each depicted xe2x80x94R group is independently selected as a substituted or unsubstituted aryl group such as those aryl substituent groups according to Formula VI below. 
where x is a number between 1 and 4 inclusive and where Rxe2x80x2 is hydrogen, a halogen atom, or is an acyl, aryl, polyaryl (fused to or separated from the aromatic ring) or alkyl radical substituent (or any combination thereof), the latter three of which may be fluorinated but are preferably non-fluorinated and may be straight-chained, branched, cyclic. The xe2x80x94Rxe2x80x2 group may optionally contain one or more catenary heteroatoms, that is, a non-carbon atom such as nitrogen or oxygen. It will be understood from the above formula that the constituent xe2x80x94Rxe2x80x2 group can be attached in any position in the ring relative to the bond attaching it to the oxalate group depicted in Formula V.
Useful alkyl groups (R in the above formula) include alkyl groups having from 2 to 20 carbon atoms. The alkyl groups may be cyclic or acyclic, linear or branched, fluorinated or non-fluorinated, may be un-substituted or may be substituted with an aryl or one or more functional groups, and may contain one or more catenary heteroatoms. Preferred alkyl and substituted alkyl groups include ethyl, propyl, and isopropyl.
It will be understood that the oxalate-blocked compounds may be oligomerized oxalates. Oligomeric oxalates, so formed, are also useful in the practice of the invention and are considered within the scope thereof. It will be further understood that the above-depicted oxalate-blocked crosslinking agents may have only one oxalate substituent and where more than one oxalate substituent is present, that substituent may be the same or may be different in structure than the other substituent or substituents present. It will also be understood that the compositions of the invention may contain one or more oxalate blocked compounds or may contain a mixture of one or more oxalate-blocked compounds and one or more other crosslinking agents.
One type of conventional crosslinking agent for a fluorocarbon elastomer gum which may be used in combination with an oxalate-blocked crosslinking agent of the invention is a polyhydroxy compound. The polyhydroxy compound may be used in its free or non-salt form or as the anionic portion of the chosen organo-onium accelerator. The crosslinking agent may be any of those polyhydroxy compounds known in the art to function as a crosslinking agent or co-curative for fluoropolymers, such as those polyhydroxy compounds disclosed in U.S. Pat. Nos. 3,876,654 (Pattison) and 4,233,421 (Worm). Representative aromatic polyhydroxy compounds include any one of the following: di-, tri-, and tetrahydroxybenzenes, naphthalenes, and anthracenes, and bisphenols of the following formula: 
wherein A is a difunctional aliphatic, cycloaliphatic, or aromatic radical of 1 to 13 carbon atoms, or a thio, oxy, carbonyl, sulfonyl, or sulfonyl radical, A is optionally substituted with at least one chlorine or fluorine atom, x is 0 or 1, n is 1 or 2, and any aromatic ring of the polyhydroxy compound is optionally substituted with at least one atom of chlorine, fluorine, bromine, or with a carboxyl or an acyl radical (for example, xe2x80x94COR where R is H or a C1 to C8 alkyl, aryl, or cycloalkyl group) or alkyl radical with, for example, 1 to 8 carbon atoms. It will be understood from the above bisphenol formula that the xe2x80x94OH groups can be attached in any position (other than number one) in either ring. Blends of two or more of these compounds are also used.
One of the most useful and commonly employed aromatic polyphenols of the above formula is 4,4xe2x80x2-hexafluoroisopropylidenyl bisphenol, known more commonly as bisphenol AF. The compounds 4,4xe2x80x2-dihydroxydiphenyl sulfone (also known as bisphenol S) and 4,4xe2x80x2-isopropylidenyl bisphenol (also known as bisphenol A) are also widely used in practice.
Other classes of crosslinking agents that may be used in the compositions of the invention are the carbonate-blocked compounds described in U.S. Pat. No. 5,728,773, the description of which is incorporated by reference herein, and the monohydroxy functional phenol compounds described in U.S. Pat. No. 5,756,588, the description of which is incorporated by reference.
Fluoroaliphatic sulfonamides can also be added to the compositions of the invention, including those of the formula RfSO2NHRxe2x80x3, where Rxe2x80x3 is an alkyl radical having, for example, from 1 to 20 carbon atoms, preferably from 1 to 12 carbon atoms, Rf is a fluoroaliphatic radical such as a perfluoroalkyl, for example, CnF2n+1 where n is 1 to 20, or perfluorocycloalkyl, for example, CnF2nxe2x88x921 where n is 3 to 20, such compounds being described, for example, in U.S. Pat. No. 5,086,123 (Guenther et al.). The fluoroaliphatic sulfonamide is preferably a perfluoroalkylsulfonamide and may be added as a separate compound, or as the anion of the organo-onium compound.
Fillers can be mixed with the fluoropolymer gum to improve molding characteristics and other properties. When a filler is employed, it can be added to the vulcanization recipe in amounts of up to about 100 parts per hundred parts by weight of gum, preferably between about 15 to 50 parts per hundred parts by weight of the gum. Examples of fillers which may be used are reinforcing thermal or furnace grade carbon blacks or non-black pigments of relatively low reinforcement characteristics such as clays and barytes.
The cure accelerators and crosslinking agent or agents can be added to the uncured polymer gum in the form of finely divided solids or as solutions in alcohol or ketone solvents by mixing the materials into the polymer gum stock. Thus mixed, the gum stock can generally be stored at room temperature for extended periods of time.
Prior to curing, an acid acceptor is mixed into the gum stock, after which storage life of the stock is more limited. Acid acceptors can be inorganic or blends of inorganic and organic. Examples of inorganic acceptors include magnesium oxide, lead oxide, calcium oxide, calcium hydroxide, dibasic lead phosphite, zinc oxide, barium carbonate, strontium hydroxide, calcium carbonate, etc. Organic acceptors include epoxies, sodium stearate, and magnesium oxalate. The preferred acid acceptors are magnesium oxide and calcium hydroxide. The acid acceptors can be used singly or in combination, and preferably are used in amounts ranging from about 2 to 25 parts per 100 parts by weight of the polymer gum stock. All of the components of the curing system may be admixed prior to their incorporation into the polymer gum stock without departing from the scope of the invention.
The relative amounts of the crosslinking agent or agents (that is, the chosen total amount of aryl, alkyl, or allyl oxalate along with conventional crosslinking agents, if any) and onium salt are present in the composition in such amounts as to provide the desired cure and/or mold release of the composition when mixed with acid acceptor. Representative proportions of components of the curing system are as follows:
All amounts are given in parts per 100 parts polymer gum stock (abbreviated xe2x80x9cphrxe2x80x9d) or in millimoles per hundred parts polymer gum stock (abbreviated xe2x80x9cmmhrxe2x80x9d). It will be understood that these proportions are general ranges; the particular amount for each particular cure time and temperature will be apparent to one of ordinary skill in the art.
In accordance with this invention, the desired amounts of compounding ingredients and other conventional adjuvants or ingredients are added to the unvulcanized fluorocarbon gum stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as internal mixers, (for example, Banbury mixers), roll mills, or any other convenient mixing device. For best results, the temperature of the mixture on the mill typically should not rise above about 120xc2x0 C. During milling, it is preferable to distribute the components and adjuvants uniformly throughout the gum for effective cure.
The mixture is then processed and shaped, for example, by extrusion (for example, in the shape of a hose or hose lining) or molding (for example, in the form of an O-ring seal). The shaped article can then be heated to cure the gum composition and form a cured elastomer article.
Pressing of the compounded mixture (that is, press cure) is usually conducted at a temperature between about 95xc2x0 C. and about 230xc2x0 C., preferably between about 150xc2x0 C. and about 205xc2x0 C., for a period of from 1 minute to 15 hours, typically from 5 minutes to 30 minutes. A pressure of between about 700 kPa and about 20,600 kPa is usually imposed on the compounded mixture in the mold. The molds first may be coated with a release agent and prebaked. The molded vulcanizate is then usually post-cured (for example, oven-cured) at a temperature usually between about 150xc2x0 C. and about 275xc2x0 C., typically at about 232xc2x0 C., for a period of from about 2 hours to 50 hours or more depending on the cross-sectional thickness of the article. For thick sections, the temperature during the post cure is usually raised gradually from the lower limit of the range to the desired maximum temperature. The maximum temperature used is preferably about 260xc2x0 C., and is held at this value for about 4 hours or more. The compositions of this invention can be used to form seals, O-rings, gaskets, etc.